SMM behaviour in a {CoIII4CoII2DyIII2} complex: Co( <scp>ii</scp> )-based thermal barrier and Dy( <scp>iii</scp> ) spectator role

We report the synthesis, structure, and magnetic properties of a novel Co/Dy heterometallic complex, [CoIII4CoII2DyIII2(tea)₄(CH₃COO)₄(OH)₂(NO₃)₄]·2H₂O (1), the first structurally characterized example of a mixed-valent {CoIII4CoII2DyIII2} motif. It comprises two butterfly-like {CoIII2Co^IIDy^III} units linked by carboxylate and alkoxide bridges. X-ray diffraction shows unique Dy(III) and Co(II) sites, with Dy(III) nine-coordinated and Co(II) in a rhombic tetrahedral geometry. Magnetic data and quantum chemical calculations reveal weak Co(II)-Co(II) antiferromagnetic exchange, negligible Co(II)-Dy(III) interactions, and moderate Co(II) zero-field splitting (<i>D</i> ≈ 12-20 cm^-1), which drives single-molecule magnet behavior below 14 K. AC studies indicate multiple tunneling pathways and an Orbach barrier (<i>U</i>_eff ≈ 65 cm^-1) arising from Co(II), while Dy(III) ions remain largely magnetically decoupled.